Question: Which of these numbers is prime? ${2,\ 22,\ 63,\ 85,\ 93}$
A prime number has exactly two factors: 1 and itself. The factors of 2 are 1 and 2. The factors of 22 are 1, 2, 11, and 22. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 85 are 1, 5, 17, and 85. The factors of 93 are 1, 3, 31, and 93. Thus, 2 is a prime number.